This invention relates to the calcination of carbonaceous materials, particularly petroleum coke such as intended to provide carbon for making electrodes or the like. Calcining operations of this sort are commonly performed in a rotary kiln into which the green petroleum coke in suitable particulate form is fed at one end, for delivery of treated product at the other end. In the kiln, the coke is calcined at high temperature, to drive off the volatiles and moisture and shrink the coke to a predetermined, desired density. The calcined product is useful for carbon elements and structures, notably for various situations of electrical function, such as in high temperature electrochemical operations, and most particularly for anodes and lining compositions in aluminum reduction cells.
The calcining process requires adequate heating for a desirably high production rate of calcined coke, while at the same time the heating is very preferably achieved inside the kiln without substantial combustion of the carbon itself. As will be understood, the green, granular coke entering the feed end of the tubular kiln flows down the kiln at a rate depending mainly on the kiln slope, for example falling 0.5 inch per foot of run from feed end to discharge end, on diameter, for example from 6 to 15 feet, and on the kiln speed of rotation, for example in the range of 0.5 to 3 r.p.m. Provision is made for supplying heat by firing with oil or natural gas burners into the lower end of the kiln, i.e. through the hood which includes the latter. The hot products of combustion, being flame and burned gas, are thus projected into the kiln, where the hot gases flow countercurrently to the descending coke bed.
In past practice, a considerable amount of heat has also been obtained by burning the released volatile materials from the coke, i.e. consuming a substantial quantity, although by no means all, of such volatiles. Air for combustion can be supplied in part from the lower end of the kiln, where such air is introduced with or separately from the fuel, but a considerable amount of air for the combustion processes utilizing the volatiles can be introduced in central regions of the kiln, using one or more fans or blowers mounted on the exterior of the kiln shell (and rotating with it) that supply air through nozzles or ducts opening into the interior of the kiln. The excess of volatiles and the combustion products of the described burning operations are carried off in the countercurrent flow of all gases, out the gas discharge which is the coke feed end of the kiln, and commonly the excess of volatiles is used for burning elsewhere, to recover heat, e.g. with a steam boiler.
As thus explained, it has been past practice to provide heat from two sources, particularly including the fuel-fired burner at the coke discharge end of the kiln, which conventionally provides a substantial percentage of the required heat energy. Whether the supplemental fuel burner has operated continuously or in some instances intermittently (being turned off for possibly recurring times when temperature conditions have temporarily reached a sufficient point), reliance has nevertheless been placed on the supplemental fuel, as representing in a sense the primary source of at least about one third and often more than half of the required heat for calcination.
As explained above, the desired result involves removing from the charge of green petroleum coke all moisture and nearly all volatile matter while at the same time (at least in part as a separate result of heating) altering the physical nature of the coke especially by increasing its real density. More specifically, the desired physical change in the coke includes removal of moisture and volatiles, as stated, and an increase of real density e.g. up to about 2.1 g/cc (grams per cubic centimeter) and likewise an improvement in average crystallite size up to 35 Angstroms, it being understood that the mean crystallite thickness of green petroleum coke may be less than 18 Angstroms.
Although effective calcining of petroleum coke can be achieved by past practice, unstable conditions have often occurred at high production rates and control has therefore been difficult when trying to obtain such rates with desired characteristics of density and volatile removal. Past practice, for best results, has tended to consider that the heating, combustion of volatiles, and effectiveness of calcination require the condition of maximum temperature of the coke to exist at or persist to a locality very close to the coke discharge end of the kiln. As indicated above, if there is significant combustion of the carbon, as might be occasioned with greater supply of air for combustion of fuel or volatiles, the production rate of useful coke suffers. On the other hand, insufficient combustion of volatiles or fuel may fail to reach the desired results or may again limit production by requiring reduction of green coke feed rate in order to permit the available heat to reach proper calcination conditions and temperature.
Experience has also indicated that the usual procedures can sometimes be erratic and the hoped-for results of good production rate and fully calcined product are sometimes difficult to obtain consistently where the nature of the green coke varies, especially in its content of volatiles as is often the case. Procedures have been developed for making some measurements of temperature rather effectively near the discharge end of the kiln, or by observing some of the characteristics of the operation, somewhat further inside, e.g. visually or by television, yet the desired factors have remained difficult to control. Very careful attention has been needed to reach full calcination in even moderately efficient manner, and especially to account for erratic conditions or to avoid erratic results.